1. Technical Field
The present invention relates to a method and apparatus for enhancing the auditory capability of hearing impaired persons. More particularly, the present invention relates to improving the binaural hearing response of hearing impaired persons.
2. The Prior Art
It has long been recognized that the optimal method of improving the hearing response of hearing impaired individuals is to improve their binaural hearing capability. Examples of prior art devices which attempt to improve binaural hearing may be found in U.S. Pat. Nos. 3,894,196 (Briskey), 3,509,289 (Briskey et al) and 2,930,858 (Hollingsworth). In addition, the subject is discussed in considerable detail in Chapter 41 of the "Handbook of Clinical Audiology", edited by Katz, which chapter is entitled "Binaural Hearing Aids and New Innovations", by Robert A. Briskey, and which appears at pages 501-507 of the text. The latter text book chapter is expressly incorporated herein by reference, in its entirety, insofar as it discloses background material and represents the state of the art.
Binaural auditory enhancement involves using two separate microphones, each feeding its received signal through its own channel or processing circuit, to deliver the separately received signals to respective earphones. The above-referenced Briskey textbook chapter points out that binaural hearing is considerably more sensitive than monaural hearing. In addition, binaural hearing, partly because of the slight phase difference between the received signals caused by microphone spacing, significantly improves sound direction discrimination. The prior art has attempted to use these characteristics of binaural hearing in hearing aids of the head-worn or body-attached type, such as hearing aids built into eyeglasses, on-the-ear aids, pocket clip-on devices or in-the-ear instruments. Such head-worn or body-attached hearing aids, however, have generally sacrificed function for cosmetics and have, therefore, not been able to take full advantage of the auditory enhancement provided by a binaural microphone system. More specifically, the hearing aid microphones, being located on the headworn or body-attached device, are relatively remote from the source of the sound of interest. As a consequence, considerable gain is required in the individual channels to boost the received signals to a usable level. This high gain arrangement, in turn, causes considerable discomfort when sudden loud sounds are picked up in one or the other microphone. In addition, feedback ringing or squealing can often result if the microphone is not properly acoustically insulated from its corresponding earphone. Proper insulation adds considerably to the expense and complexity of the device.
Apart from the remote location of the microphone relative to the sound source, another reason for the failure of prior art binaural hearing aids to take full advantage of the improvements inherent in binaural systems concerns acoustic reflections resulting from microphone mounting arrangements. Specifically, microphones are often mounted to project from the surface of the supporting structure. Reflections from the mounting structure surface which reach the microphone are often 180.degree. out of phase with the unreflected received signals, thereby producing nulls in the channel response. Moreover, phase differences between the reflected and unreflected picked up sounds can often mask the phase difference required between the channels in a binaural system.
The aforementioned Briskey et al patent discloses a binaural hearing aid which is intended to maintain or enhance the inter-aural level differential between the contralateral and ipsilateral sounds received at the furthest and nearest ears, respectively, with respect to the sound source. Briskey et al recognize that this difference in sound level, although minute, is important because it is part of the process which permits a listener to perceive relative direction of sound. The problem with devices used prior to Briskey et al is that the separate automatic gain control (AGC) circuits employed in each channel adjusted the gain in a manner which is inversely proportional to the input signal level in that channel, with the result that the inter-aural level difference was largely eliminated. Briskey et al solved this problem by providing interchannel control signals for the respective AGC circuits to maintain the inter-aural level difference. This technique works quite well in an ideal system, but it does not accommodate changes in individual circuit components resulting from aging, wear and tear, etc., which changes cause the AGC circuit response to change in a manner which cannot be predicted at the time of manufacture. Moreover, the use of two AGC circuits results in greater expense and the need for more space than would be required by a single AGC circuit.